The present invention relates to a method of making annular fiber structures, in particular preforms for manufacturing annular parts of composite material.
A particular but non-exclusive field of application of the invention lies in making annular preforms for the manufacture of brake disks or clutch disks out of composite material, and in particular carbon-carbon (C/C) composite material.
Annular parts of composite material, such as brake disks or clutch disks, are constituted by a fiber reinforcing structure or "preform" which is densified by a matrix. For C/C composite disks, the preform is made of carbon fibers or of fibers made of a carbon precursor which is transformed into carbon by heat treatment after the preform has been made. A particular carbon preform that is available in fiber form is pre-oxidized polyacrylonitrile (PAN). The preform can be densified either by a liquid-impregnation process using a liquid precursor for carbon, e.g. a resin, and then transforming the precursor by heat treatment, or else by chemical vapor infiltration, or indeed by calefaction. For calefaction, the preform is immersed in a matrix-precursor liquid and the preform is heated, e.g. by contact with an induction core or by direct coupling with an induction coil, so that the precursor is vaporized on making contact with the preform and can infiltrate to form the matrix by depositing within the pores of the preform.
A well known method of making fiber preforms for parts made of composite material consists in superposing and needling together layers or plies of a two-dimensional fiber fabric. By way of example, the fiber fabric can be a woven cloth. The cloth may optionally be covered in a web of fibers for producing the fibers that are suitable for being displaced by needles through the superposed plies; this applies in particular when the cloth is made of fibers that are difficult to needle without being broken, and in particular carbon fibers. Such a method is described in particular in documents FR-A-2 584 106 and FR-A-2 584 109 respectively for making preforms that are plane and for making preforms that are bodies of revolution.
An annular preform for a disk can be cut out from a thick plate made up of layers that have been superposed flat and needled together. The loss of material then amounts to nearly 50% which, for preforms made of carbon fibers or of carbon precursor fibers, constitutes a very large expense.
In order to reduce this loss, proposals are made in document EP-A-0 232 059 to build up a preform by superposing and needling together annular layers, each of which is formed by assembling together a plurality of sectors. The sectors are cut out from a two-dimensional fabric. The loss of material is less than when cutting out entire rings, but it is still not negligible. In addition, the method is rather difficult to implement and to automate, in particular because of the need to position the sectors correctly while ensuring that they are offset from one layer to another so as to avoid superposing lines of separation between sectors.
It might be envisaged that annular preforms could be cut out from sleeves made by rolling a strip of cloth onto a mandrel while simultaneously needling it, as described in above-mentioned document FR-A-2 584 107. That method is relatively easy to implement without significant loss of fiber material. However, in an application to friction disks, and contrary to the other embodiments described above, the plies of the preform are then disposed perpendicularly to the friction faces, and in some cases that configuration is not optimal.
Another known technique for making fiber preforms for annular parts made of composite material consists in using a textile product in the form of a helical or spiral strip, which product is wound as flat superposed turns. The textile product can be a woven cloth made up of helical warp threads and of radial weft threads.
As described in documents FR-A-2 490 687 and FR-A-2 643 656, the spiral helical shape is given to the cloth by making use of a frustoconical roller for the warp threads being reeled out from individual spools mounted on a creel. In a cloth made in that way, the spacing between the radial weft threads increases across the width of the helical cloth between the inside diameter and the outside diameter.
In order to conserve a substantially uniform nature for the cloth across its entire width, it is proposed in the two above-mentioned documents to introduce additional weft threads that extend over a portion only of the width of the cloth, starting from its outside diameter. That solution gives rise to significant extra cost in manufacturing the cloth, and is a non-negligible source of defects. Another solution, described in patent application FR 95 14 000, consists in increasing the mass per unit area of the warp of the helical cloth between the inside diameter and the outside diameter thereof so as to ensure that in terms of density per unit volume of the preform, the decrease in weft density is compensated approximately. Although less expensive than increasing the density of weft fibers towards the outside diameter, that solution nevertheless remains rather complex since it requires the use of warp threads of varying weight and/or varying mass per unit area between the inside diameter and the outside diameter of the cloth.
In yet another known technique, fiber preforms for annular parts made of composite material, and in particular for brake disks, are made by winding flattened tubular braids helically. The tubular braids can be rectilinear, as described in document EP-A-0 528 336. The braids are then deformed so as to be wound into a helix. Longitudinal threads can be added during manufacture of the braid so as to improve the dimensional stability of the preform and so as to compensate for variation in density per unit area between the inside diameter and the outside diameter of the wound flattened braid. Proposals have also been made in document EP-A- 0 683 261 to use helical tubular braids. That makes it possible to overcome the limits on deformability of rectilinear tubular braids when they are being wound into a helix. Nevertheless, the variation in density per unit area still needs to be compensated by adding longitudinal fibers or by juxtaposing a plurality of flattened braids of small width between the inside diameter and the outside diameter. Those solutions make preform manufacture relatively complex, and thus expensive, without providing a completely satisfactory solution to the problem of variation in density per unit area.